The present application relates to a lithium ion battery using, for example, a carbon material as the anode.
The significant development of portable electronic technologies in recent years has permitted of the recognition of electronic devices such as portable telephones, laptop computers and personal digital assistants (PDAs) as fundamental technologies supporting a high degree of information-oriented society. Also, research and developments concerning high functionalization of these devices are being energetically made, and the power consumption of electronic devices is steadily increasing in proportion to this. On the other hand, it is demanded of these electronic devices to work for a long time and it has been inevitably desired to develop secondary batteries having a high energy density which are driving power sources.
The energy density of a battery provided within an electronic device is preferably higher from the viewpoint of the occupied volume and weight of the battery. In order to cope with this demand, there is a proposal of a secondary battery using lithium Li as an electrode reactive material. Among these secondary batteries, a lithium ion secondary battery using a carbon material that can be doped or dedoped with a lithium ion as the anode has come to be provided within almost all devices because it has a high energy density.
However, such a battery has been already utilized for charging or discharging up to a range close to the theoretical capacity of a carbon material. For this, studies as to measures taken to raise energy density are being made to increase the thickness of an active material layer, thereby increasing the ratio of the active material layer and decreasing the ratio of a current collector and a separator as shown in Japanese Patent Application Laid-Open (JP-A) No. 9-204936.
However, the diffusion of lithium ions in the anode is insufficient in batteries improved in energy density and therefore, measures taken to improve the diffusion of lithium ions are strongly desired. In the case of, particularly, increasing the thickness of the active material layer, the area of the electrode is decreased because the length of the electrode is decreased to manufacture a battery having the same size. Therefore, these batteries have the problem that the charge density loaded on the anode is more increased during charging than in the case of batteries in related art and the diffusion of lithium and electric acceptance of lithium in the anode is unable to catch up with the increase in charging density, with the result that a lithium metal tends to precipitate. The precipitated lithium metal tends to peel off or fall down, leading to a reduction in active material, which results in significantly deteriorated cycle characteristics. It is therefore difficult to increase the thickness of the active material layer.
In light of this, there is a proposal concerning a lithium ion secondary battery improved in the diffusibility of lithium in an anode by mixing ceramic particles in the anode as shown in JP-A No. 10-255807.
In JP-A No. 10-255807, the internal resistance of the electrode is reduced by mixing ceramic having high ion conductivity in the anode to thereby obtain a lithium ion secondary battery having a high capacity. Also, the strength of the electrode can be improved at the same time and it is therefore possible to improve cycle characteristics. In JP-A No. 10-255807, there is the description that the performance of the battery can be improved by compounding the above ceramic in an amount of 0.01 to 20 parts by weight for 100 parts by weight of the anode active material. At this time, ceramic having 10 μm or less is used as the above ceramic.
When ceramic is mixed in the anode as shown in JP-A-No. 10-255807, there is a fear that no ion diffusing effect is obtained if the particle diameter of the ceramic to be mixed is too large in a lithium ion battery using an electrolyte solution containing an organic solvent. Also, there is a fear that the diffusion of ions is hindered.
Generally, in lithium ion batteries, a part of the electrolyte solution is dissolved when they are charged in the initial charging, whereby an organic SEI (Solid Electrolyte of Interface) film is formed on the surface of the anode active material. This organic SEI film causes a rise in the interfacial resistance of the anode active material to thereby hinder the diffusion of ions. In order to solve this problem, ceramic particles are mixed in an anode to form an anode active material layer. It is therefore possible to form a complex SEI film increased in ion diffusibility which is provided with not only the organic SEI film but also an organic SEI film having high ion diffusibility in which ceramic besides the organic SEI film is stuck to the surface of the anode active material. In such a complex SEI film, lithium ions can be moved at a high rate on the surface of ceramic in the film and it is therefore possible to improve the diffusibility of ions on the surface of the anode active material.
However, the organic SEI film formed on the surface of anode active material has a thickness of about several nm. Therefore, when ceramic having a large particle diameter is mixed, the ceramic surface facing the electrolyte solution is increased resultantly. Because the ionic diffusibility of the surface of ceramic is lower than that of the electrolyte solution, there is a fear that the diffusion of ions is hindered if the particle diameter of ceramic is too large. Also, the surface area of ceramic which contributes to ionic diffusion in the complex SEI film is so reduced that the effect of improving the ionic diffusibility on the surface of the anode active material is reduced, with the result that it is difficult to limit the precipitation of lithium.